In the information society, it has become increasingly important to communicate using an optical signal to quickly transmit and receive a larger amount of information. A communication system using such an optical signal may comprise a transmitter, a receiver, and an optical fiber connecting the transmitter and the receiver. In the communication system using the optical signal, the transmitter converts an electrical signal to an optical signal in one of two states “1” and “0” corresponding to the light strength, and the receiver converts the optical signal back to an electrical signal in order to transmit information. Measurement of the strength of the optical signal is important for the receiver in order to accurately read the information indicated by the optical signal.
The receiver in the communication system using the optical signal may comprise: a photodiode that converts the optical signal to a current signal; a unit that measures the strength of the optical signal; and a logic unit that executes a process according to the optical signal based on the current signal and the strength of the optical signal. A potential difference at a stable magnitude is necessary at both ends of the photodiode in order to convert the optical signal to the current signal. The receiver may prevent a circuit connected to a serial part of the photodiode from affecting the potentials at both ends of the photodiode. The photodiode may generate a current signal in a significantly wide range from several μA (microampere) to several mA, and thus means for preventing the circuit connected to the serial part of the photodiode from affecting the potentials at both ends of the photodiode needs to handle a wide range of current signal generated by the photodiode.
It is known that two or more circuits that operate in a current mode are connected to each other, and a current mirror circuit is provided between a circuit of a former part and a circuit of a serial part to prevent the circuit of the serial part from affecting the circuit of the former part. Separately, a current mirror circuit using a CMOS (Complementary Metal Oxide Semiconductor) transistor used in the technique, in which a reference current is highly accurately amplified based on not only a replication of the potential difference between the gate and the source of the transistor, but also a replication of the potential difference between the drain and the source.
For example, Japanese Patent Laid-Open No. 2008-288900 (hereinafter, called “'900 Publication”) discloses a differential amplifier including: an input differential pair; cascode current mirror circuits with at least two stages connected as a load to the input differential pair; a tail current source that supplies a tail current to the input differential pair; and a constant current source connected in parallel with the input differential pair to supply a constant current to the tail current source.
For example, Japanese Patent Laid-Open No. 2002-270768 (hereinafter, called “'768 Publication”) discloses a CMOS reference voltage circuit including: first and second diode-connected transistors that are grounded and driven by two constant currents at a certain current ratio; and means for amplifying a difference voltage of output voltages of the first and second diode-connected transistors at a certain magnification and adding the voltage to the output voltage from the first or second diode-connected transistor. In the disclosed CMOS reference voltage circuit, the means for amplifying and adding includes first and second operational transconductance amplifiers (hereinafter, called “OTA”) and a current mirror circuit. An output terminal voltage of the second OTA is handled as an output voltage, and the first OTA inputs the difference voltage. A normal phase input terminal of the second OTA receives the output voltage of the first or second diode-connected transistor, and a reverse phase input terminal receives the output voltage of the second OTA. The CMOS reference voltage circuit is driven by a current according to an output current of the first OTA.
In the conventional differential amplifier as described above, the cascode current mirror circuits are used to highly accurately amplify the input current of the differential amplifier while preventing an increase in the chip area. However, the voltage applied to the input differential pair is reduced by the potential difference between the sources and the gates of the cascode-connected transistors when the input current is large in the differential amplifier disclosed in '900 Publication. Necessarily, the range of the input current is limited.
Furthermore, the conventional CMOS reference voltage circuit as described uses the current mirror circuit by way of using the OTA to highly accurately amplify the input current while eliminating the effect of the cascode connection on an element on the input side. However, the OTA requires a large chip area to handle a wide range of input current in the CMOS reference voltage circuit disclosed in '768 Publication.